The major objective of this project continues to be to define factors, other than divalent cations, which affect parathyroid hormone (PTH) secretion. We wish to better define the interrelationships between vitamin D (and its various metabolites) and PTH. We are particularly interested in elucidating the possible role of 24,25(OH)2D3 on bone calcium accretion. Further we plan to investigate the effects of normal aging processes on the various parameters (i.e., calcium, phosphorus, magnesium, and alkaline phosphatase) and controllers (i.e., PTH and vitamin D metabolites) of normal calcium homeostasis.